This invention relates to an electromagnetic wave filter which is to be set in front of a cathode ray tube (CRT) or a display, such as a field emission display (FED) or a plasma display panel, to cut the electromagnetic waves emitted from the display, etc. and a process for producing the same.
Because the electromagnetic wave filter of the invention is transparent to light and electrically conductive, it is also useful as a transparent conductive film. Further, it exhibits electromagnetic wave shielding performance over a broad range of wavelengths including the near infrared region and even longer wavelengths and is therefore applicable as heat-shielding window glass of buildings or automobiles.
Conventional transparent electromagnetic wave filters comprising a transparent substrate, such as a glass plate, coated with an electromagnetic wave shield film include those comprising a transparent substrate having alternately formed thereon dielectric layers, such as transparent metal oxide layers, and a silver layer(s).
JP-A-5-42624 discloses a glass plate coated with an electromagnetic wave shield film having a silver layer sandwiched in between a pair of laminates of dielectric layers, showing in Example 2 an electromagnetic wave shield film having a laminate structure of (substrate)/ZnO/SnO2/ZnO/SnO2/ZnO/Ag/ZnO/SnO2/ZnO/SnO2/ZnO (n=1). In this Example the metal oxide layers ZnO and SnO2 are formed by oxygen-reactive sputtering using ZnO and SnO2 as respective targets and a reactive gas comprising argon and oxygen.
JP-A-9-85893 proposes a glass plate coated with an infrared ray shield film having a laminate structure in which a dielectric layer and a silver layer are alternately laminated with each other. For example, Example 11 shows an infrared ray shield film having a 5-layered laminate structure of (glass)/Al-containing ZnO/Pd-containing Ag/Al-containing ZnO/Pd-containing Ag/Al-containing ZnO (two silver layers are used; n=2), and Example 13 describes a laminate structure in which the outermost dielectric layer is a laminate of an Al-containing ZnO layer and an Si-containing SnO2 layer. Each of these dielectric layers is formed by oxygen-reactive sputtering using the respective metal constituting the metal oxide layer as a target and oxygen or an oxygen/argon mixed gas as shown in Table 1.
JP-A-8-104547 teaches heat-insulating glass comprising a glass plate having formed thereon an infrared ray shield film. The infrared ray shield film is a laminate of a silver layer and a metal oxide layer as exemplified in Tables 1 and 6. For example, Example 13 demonstrates a 5-layered laminate structure of (glass)/ZnO:Al2O3/Ag/ZnO:Al2O3/Ag/ZnO:Al2O3 (two silver layers are used; n=2). The ZnO:Al2O3 layer is formed by sputtering using a sintered body comprising ZnO containing 2% by weight of Al2O3 as a target.
Highly bright image display of a plasma display panel can be achieved by a powerful plasma discharge in a discharge cell. Therefore, the plasma display panel emits near infrared rays as well as electromagnetic waves from the discharge cell toward the front of the panel. It has been pointed out that electromagnetic waves have a possibility of giving adverse influences to a human body. Further, the infrared rays emitted may be detected by the receptor of a remote control of an appliance near the plasma display, tending to cause an incorrect action of turning the switch on.
To overcome the above problems, it has been suggested to provide a transparent material having electromagnetic wave shielding performance in front of the plasma display panel. For this purpose, it has been a practice that an electromagnetic wave filter comprising a glass plate having on one side thereof an alternate laminate of dielectric layers and a silver layer(s) is attached to the front of a plasma display. Such an electromagnetic wave filter is required to satisfy the following performance requirements.
(1) A shielding effect against electromagnetic waves (i.e., low sheet resistance of the electromagnetic wave shielding film);
(2) a low transmission for heat rays of the near infrared region (region of from 800 to 900 nm), which are used for remote control of appliances;
(3) a high visible light transmission to secure a bright image display;
(4) sufficient durability against moist heat, etc. for use as exposed in air; and
(5) freedom from pin holes or contamination with foreign matter.
Aiming at settlement of all the outstanding problems, the concept of the present invention lies in that the requirements (1) through (4) are satisfied by designing the laminate structure of dielectric layers and a silver layer(s) to have the characteristics of a so-called optical low path filter, i.e., to have a high visible light transmission and a low near infrared transmission, showing a sharp change in transmission at the border of the two regions. The concept also resides in that the requirement (5) is met by designing a novel layer as the dielectric layers of the laminate.
As mentioned above, the electromagnetic wave shield film disclosed in JP-A-5-42624 supra has one silver layer sandwiched between laminates of dielectric layers. The silver layer, which secures electromagnetic wave shielding performance, should have an increased thickness to exhibit satisfactory performance, but such results in a visible light transmission loss. Therefore, the requirements (1), (2) and (3) are not to be satisfied simultaneously. Moreover, because the ZnO layer and SnO2 layer constituting the dielectric layers are electrically insulating, it has been difficult to obtain glow discharge plasma in a stable manner in forming these layers by sputtering, only to provide a film suffering from pinholes or adhesion of foreign matter.
The laminate disclosed in JP-A-9-85893 supra, which comprises Al-containing ZnO layers and Pd-containing Ag layers, accomplishes improvements on the requirements (1) and (3) owing to the two silver layers but still fails to fulfil the requirement (2), i.e., an erroneous action preventive function. That is, the laminate has insufficient performance to be used as an electromagnetic wave filter in front of a plasma display panel. Additionally, because the dielectric layer comprising a metal oxide is formed by oxygen-reactive sputtering by using the corresponding metal as a target and oxygen gas or a mixed gas of oxygen and argon gas, the electrically insulating metal oxide is deposited on the substrate and near the surface of the target. It tends to follow that charges stay on the surface of the target and of the substrate to cause sparks or arcs, failing to obtain a stable glow discharge in the thin film formation. As a result, the metal oxide layer can suffer from pin holes due to microsparks or adhesion of foreign matter during the film formation.
The infrared ray shield film of JP-A-8-104547 supra, which also contains two silver layers, meets difficulty in satisfying the requirements for high electromagnetic wave shielding performance, low near infrared ray transmission, and high visible light transmission for the same reasons as mentioned above. The dielectric layers of the laminate are layers mainly comprising zinc oxide which are formed by oxygen-reactive sputtering using a mixture of Al2O3 and 2% by weight of ZnO as a target. However, the dielectric layers formed on the substrate have high electric resistance so that the surface undergoes electrification during film formation, which can result in pin holes.
An object of the present invention is to solve the above-described problems associated with conventional techniques and to provide an electromagnetic wave filter having performance properties required of a practical and high-performance filter.
The object is accomplished by a light transmitting electromagnetic wave filter comprising a transparent substrate having on one side thereof a light transmitting electromagnetic wave shield film comprising a laminate of 2n+1 (nxe2x89xa71) layers in which at least two dielectric layers and at least one silver layer are alternately laminated with each other with the dielectric layer being the first to be provided on the transparent substrate, wherein at least one of the dielectric layers is an antistatic metal oxide layer having a refractive index of 1.6 to 2.7 at a wavelength of 550 nm.
The dielectric layer of the laminate has antistatic properties, while the silver layer is conductive. Therefore, charges are prevented from staying on the electromagnetic wave shield film while the film is formed on the substrate. Thus, abnormal discharges can be suppressed while laminating a silver layer with a dielectric layer by sputtering, and there is obtained a laminate hardly suffering from pin holes resulting from an abnormal discharge or contamination with foreign matter.
The metal oxide layer constituting the dielectric layer has a refractive index of 1.6 to 2.7 at 550 nm. If the refractive index is smaller than 1.6, the effect of light interference in improving transparency to visible light will be lessened, making it difficult to obtain an increased visible light transmission. For ensuring the improvement in visible light transmission, a preferred refractive index of the metal oxide layer is 1.9 or higher. The upper limit of the refractive index of the metal oxide la at 550 nm is 2.7 times that of titanium dioxide, which has the highest refractive index among metal oxides.